Coating processes in which holders line those of the present invention are used include dipping processes in which liquid paints and other coatings are applied to workpieces by immersion, and also fluidized bed coating, spray painting, electroplating and anodizing processes.
Where numerous substantially identical workpieces are to be subjected to such a process, the workpieces are usually transported to, through and away from the coating station on a carrier such as a chain conveyor that is equipped with numerous workpiece holders arranged at intervals along its length. A workpiece is loaded onto each holder at a loading station ahead of the coating station and is removed from the holder at an unloading station down-line from the coating station. Loading and unloading are usually manual operations, and therefore the holders that support the workpieces must be so configured that those operations can be performed very quickly and easily by persons having minimal skill and dexterity.
The holders heretofore used for connecting workpieces to the carrier have usually been formed as a hook or loop bent from a single piece of relatively stiff wire. The low cost of such holders was an important consideration because a workpiece holder tends to become coated after carrying one or a few workpieces through the coating station and must then be removed from the carrier, discarded and replaced with a new one. Obviously it is also important that the holder be arranged for quick and easy installation on the carrier and removal from it.
A type of holder heretofore used for many coating operations, and especially for those involving apertured workpieces, was essentially in the form of a simple hook having an eye or loop at its top by which it was hung on a carrier and having a bowed bottom that projected through an aperture in the workpiece. In some cases the holder was arranged to support a number of workpieces and was in the form of a wire rack or frame having a number of hooks for supporting workpieces in spaced apart relation to one another and having a pair of hooks at its top that provided for attachment of the holder to a carrier.
Such prior holders have been satisfactory in most respects, but they have had some defects and disadvantages which have apparently been accepted as inevitable in the sense that there was no obvious way to overcome or avoid them without sacrificing the important characteristics of low cost, quick and easy loading and unloading, and quick and easy installation on the carrier and removal from it.
A major defect of many prior holders was that they provided only single-point support for each workpiece, so that the workpiece was free for at least a limited amount of turning and swinging. As a result, there was no assurance that the workpiece would enter and pass through the coating station in an optimum attitude such that it would assuredly receive an even and uniform coating. In some cases, particularly where the workpiece was immersed in a liquid bath at the coating station, the workpiece tended to float off of the holder at that station as its surface encountered the surface of the liquid in which it was to be dipped.
To some extent these disadvantages could be overcome if the workpiece had two or more apertures, since the holder could then be in the form of a rack having a pair of hooks, one for each of two apertures in the workpiece. In such cases, however, loading of workpieces onto the holders tended to be complicated and slowed by the need for manipulating each workpiece to bring first one of its apertures and then the other into alignment with the respective holder hooks intended for them.
An important disadvantage common to all such prior holders was apparent when they were employed in a coating process which required each workpiece to be connected in an electrical circuit, as in an electroplating process or an electrostatic spray painting process. In such cases the holder was relied upon to connect the workpiece in the electrical circuit, but the electrical connection between the holder and the workpiece was a somewhat unreliable one that tended to offer a high resistance to current flow inasmuch as the hook or hooks of the holder had only limited point contact with the workpiece and bore against it with a force that depended upon the weight of the workpiece.